The field of the present invention pertains equipment and machines for semiconductor fabrication processing. More particularly, the present invention relates to equipment and machines for CMP (chemical mechanical planarization) of semiconductor wafers.
Most of the power and usefulness of today""s digital IC devices can be attributed to the increasing levels of integration. More and more components (resistors, diodes, transistors, and the like) are continually being integrated into the underlying chip, or IC. The starting material for typical ICs is very high purity silicon. The material is grown as a single crystal. It takes the shape of a solid cylinder. This crystal is then sawed (like a loaf of bread) to produce wafers typically 10 to 30 cm in diameter and 250 microns thick.
The geometry of the features of the IC components are commonly defined photographically through a process known as photolithography. Very fine surface geometries can be reproduced accurately by this technique. The photolithography process is used to define component regions and build up components one layer on top of another. Complex ICs can often have many different built-up layers, each layer having components, each layer having differing interconnections, and each layer stacked on top of the previous layer. The resulting topography of these complex IC""s often resemble familiar terrestrial xe2x80x9cmountain ranges,xe2x80x9d with many xe2x80x9chillsxe2x80x9d and xe2x80x9cvalleysxe2x80x9d as the IC components are built up on the underlying surface of the silicon wafer.
In the photolithography process, a mask image, or pattern, defining the various components is focused onto a photosensitive layer using ultraviolet light. The image is focused onto the surface using the optical means of the photolithography tool and is imprinted into the photosensitive layer. To build ever smaller features, increasingly fine images must be focused onto the surface of the photosensitive layer, e.g. optical resolution must increase. As optical resolution increases, the depth of focus of the mask image correspondingly narrows. This is due to the narrow range in depth of focus imposed by the high numerical aperture lenses in the photolithography tool. This narrowing depth of focus is often the limiting factor in the degree of resolution obtainable and, thus, the smallest components obtainable using the photolithography tool. The extreme topography of complex ICs, the xe2x80x9chillsxe2x80x9d and xe2x80x9cvalleys,xe2x80x9d exaggerate the effects of decreasing depth of focus. Thus, in order properly to focus the mask image defining sub-micron geometries onto the photosensitive layer, a precisely flat surface is desired. The precisely flat (e.g. fully planarized) surface will allow for extremely small depths of focus and, in turn, allow the definition and subsequent fabrication of extremely small components.
Chemical-mechanical planarization (CMP) is the preferred method of obtaining full planarization of a wafer. It involves removing a sacrificial layer of dielectric material or metal using mechanical contact between the wafer and a moving polishing pad with chemical assistance from a polishing slurry. Polishing flattens out height differences since high areas of topography (hills) are removed faster than areas of low topography (valleys). CMP is the only technique with the capability of smoothing out topography over millimeter scale planarization distances leading to maximum angles of much less than one degree after polishing.
FIG. 1 shows a side cut away view of a conventional CMP machine 100 such as the Strasbaugh 6DS-SP. CMP machine 100 typically consists of a platen 104, or turn table, covered with a polishing pad 102 that is made of resilient material. The polishing pad 102 is typically textured, often with a plurality of predetermined grooves, to aid the polishing process. The polishing pad 102 and the platen 104 rotate at a predetermined speed. Wafers 105 are held in place at the bottom ends of spindles 101 to be polished face-down. Spindles 101 are rotated by motor assembly 110 that are located within a bridge housing 120. The bridge housing 120 itself moves in a translatory motion (illustrated by arrow 130) allowing the wafers 105 to cover more of surface of the polishing pad 102. Typically, CMP machine 100 also includes a slurry dispense mechanisms for dispensing a flow of slurry onto the polishing pad 102. CMP machine 100 may also include an enclosure 140 for providing an isolated environment for CMP operations.
The slurry is a mixture of de-ionized water and polishing agents designed to chemically and mechanically smoothen and predictably planarize the wafer. The rotating action of both the polishing pad 102 and the spindles 101 and the translatory motion of the bridge housing 120, in conjunction with the polishing action of the slurry, combine to planarize, or polish, the wafers 105 such that topography over millimeter scale planarization distances is nearly completely smoothed away. Once CMP is complete, wafers 105 are removed from polishing pad 102 and are prepared for the next phase in the device fabrication process.
The rate at which the wafers 105 are planarized is generally referred to as the removal rate. A constant and predictable removal rate is important to the uniformity and performance of the wafer fabrication process. The removal rate should be expedient, yet yield precisely planarized wafers, free from surface topography. If the removal rate is too slow, the number of planarized wafers produced in a given period of time decreases, degrading wafer through-put of the fabrication process. If the removal rate is too fast, the CMP planarization process may not be uniform across the surface of the wafers, degrading the yield of the fabrication process. Thus, it is important to precisely control the removal rate.
The removal rate, however, may vary from one wafer to another. Even when the wafers are polished at the same time, unevenness on the surfaces of the wafers and the polishing pad may cause one wafer to be polished faster than another. The removal rate may also vary from one batch of wafers to another batch if the polishing pad wears down unevenly. The result is that the wafers may not be uniformly planarized.
Therefore, what is needed is an improved apparatus and methodology for performing CMP. What is further needed is an apparatus for performing CMP such that the wafers are uniformly planarized.
Accordingly, the present invention provides an apparatus for performing chemical-mechanical planarization (CMP) of semiconductor wafers with improved process window, process flexibility and cost. Particularly, the present invention allows independent micro-control of each spindle for tailored CMP performance.
The present invention provides, in one embodiment, a CMP apparatus that includes a stationary bridge that houses a rack and pinion assembly. The rack and pinion assembly is coupled to a plurality of spindle motor assemblies each of which is coupled to rotate a spindle. Significantly, translation of the spindles is achieved with the rack and pinion assembly. Further, the translation of the spindles can be independently and individually controlled. An advantage of the present independent spindle motion design allows optimization of the CMP process for each spindle and enables more accurate prediction of the effect of translation on CMP performance. Independent rotation and downforce capability of the present invention provides additional flexibility in terms of tuning polish rates and uniformity. Another advantage of the present invention is that a more compact enclosure for wafer isolation can be achieved.
According to one embodiment of the invention, the CMP apparatus includes a turn-table covered by a polishing pad; spindles operable to push wafers against the polishing pad; spindle motor assemblies coupled to the spindles and operable to rotate the wafers on the polishing pad. Translational motions of the spindles across the polishing pad are individually driven by rack and pinion mechanisms contained within a bridge housing. In the present embodiment, the rack and pinion assemblies are individually micro-controlled to achieve CMP performance that is custom tailored for the particular CMP process.